One way you could do this is to sort them by dimensions in whatever file manager you use. In Windows, for example, you can switch to detailed view, right click the Name column and add something like a "Dimensions" column. Then, select the bottom (or top) ones and cut-paste them some place else, where you can batch convert with Irfanview or whatever.

With ImageMagick you can use mogrify with the "-resize 600x800>" option. That would resize only images that are more than 600 pixel wide or more than 800 pixel high. If you don't care about the orientation (i.e., if you don't want to resize 800x600 images either), I don't know if there's an option for that, you may have to classify the images in portrait/landscape first.